Multimeter - Testing/Results

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toxictoad   1 W

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Multimeter - Testing/Results

Post by toxictoad » Jun 13 2018 6:53am

Hey,

I have a N19BW CAT III Multi Meter and a small disco light that plugged into the wall (before I removed the plug) and I want to connect it to a step down converter so I can power it from a 36v battery. I thought I knew how to get the voltage but I'm confused at the results so may have it wrong.

The plug says: Input-100v-240v AC 50/60Hz, Output 8.4v 1A

I set the MM to 600 VAC and when connected to the power wires I get a reading of 021 or 200 VAC 20.6

Am I using the wrong settings on the MM or reading it wrong? Could I just set the step down converter to 8.4v and be safe?

Thanks

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eee291   10 kW

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Re: Multimeter - Testing/Results

Post by eee291 » Jun 13 2018 9:02am

Dumb question here, but did you set the dial to VDC? Since you want to measure 8.4V DC you have to switch it to the left side.
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toxictoad   1 W

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Re: Multimeter - Testing/Results

Post by toxictoad » Jun 13 2018 1:05pm

Not a dumb question :D
I'd read to check using the AC, which is where I went wrong. On the left DC @ 600 gives a reading of 9. Should I use the same setting and adjust the step down converter to 9 also? or is there a more accurate way of doing it?

Thanks

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eee291   10 kW

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Re: Multimeter - Testing/Results

Post by eee291 » Jun 13 2018 1:14pm

I would get it as close to 8.4V as possible.

Also some (cheap) step down or boost converters don't keep the set voltage locked. For instance a 36V battery fully charged is 42V, so I set the converter to whatever I want, but as the Battery voltage drops the output voltage of the converter might also drop.

So charge your Battery fully, and then adjust to 8.4V and see if it goes down after some time.

If it's not accurate enough, then you can change the setting to 20 VDC for higher Voltage resolution.

For instance on 200VDC you might see 008.4V, but on 20VDC you see 08.45V.

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spinningmagnets   100 GW

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Re: Multimeter - Testing/Results

Post by spinningmagnets » Jun 13 2018 1:15pm

This one? (N19BW)
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toxictoad   1 W

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Re: Multimeter - Testing/Results

Post by toxictoad » Jun 13 2018 3:05pm

I get 3 different readings and none are 8.4
600 = 009
200 = 10.1
20 = 10.21

Seems odd?

This one
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eee291   10 kW

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Re: Multimeter - Testing/Results

Post by eee291 » Jun 13 2018 3:12pm

The measuring result is pretty much what I'd expect.

So, about that step down Converter. Could you show us a picture or a link from where you bought it from?

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Re: Multimeter - Testing/Results

Post by toxictoad » Jun 13 2018 3:21pm

One of these https://www.ebay.co.uk/itm/200W-15A-Vol ... 2749.l2649

I used another, the same type a couple of years ago when connecting a small amp, just forgotten what I did

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eee291   10 kW

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Re: Multimeter - Testing/Results

Post by eee291 » Jun 13 2018 3:35pm

Image
All you need to do is connect your voltmeter into the output of the Converter and start turning the golden flathead on the blue box until you hit 8.4V. :)
Last edited by eee291 on Jun 13 2018 3:38pm, edited 1 time in total.

toxictoad   1 W

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Re: Multimeter - Testing/Results

Post by toxictoad » Jun 13 2018 3:37pm

Fantastic thank you!

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Re: Multimeter - Testing/Results

Post by toxictoad » Jun 15 2018 5:32am

I connected the light today via the buck converter with the output set to 8.40 (20V DC) and after a couple of minutes of being on, I could smell burning plastic and see very slight smoke coming from the light :shock:

I am a bit confused.

Why does the MM show higher voltage than what the plug says, is this normal and what would I do if the plug didn't show voltage ratings at all?

I seems that it's getting to much power, which is causing the overheating?

Any advice?
Thanks

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eee291   10 kW

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Re: Multimeter - Testing/Results

Post by eee291 » Jun 15 2018 5:59am

What is (20VDC) supposed to mean?

I thought it needed 8.4VDC.
I'm so confused right now :?:

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Re: Multimeter - Testing/Results

Post by toxictoad » Jun 15 2018 6:05am

To get as accurate as poss I set the multimeter to 20V DC and this gives me the reading of 8.40v from the output on the converter.

I've left it outside on concrete slabs but it seems to be ok now so maybe the smoke was because the light was new? it was a cheapo from china...I'll leave it on an hr or so and see what goes on

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Re: Multimeter - Testing/Results

Post by toxictoad » Jun 15 2018 8:04am

After running for over an hour everything looked fine. I unplugged and took the lamp apart to see inside and I can see no damage to anything so I think it's good to go.

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eee291   10 kW

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Re: Multimeter - Testing/Results

Post by eee291 » Jun 15 2018 8:16am

Ah, i see. Good to hear it didn't burn something down.

By the way there are cheapo bike lamps that can take 12-60V on eBay or AliExpress.

I bought one that can do 100V for my budget 72V moped build, and it works great.

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Re: Multimeter - Testing/Results

Post by toxictoad » Jun 16 2018 12:15pm

Creating a mobile disco :D trying to work out if I can fit it all into a metal flight case I have. Be looking at 2 disco type lights/lasers & a strobe and for the sound a 800w poweramp and possibly putting the speakers in the lid with a couple that are stand alone from an old 5.1 PC setup.

I am still confused as to why the MM readings from the plug don't match 8.4v and why I would test the voltage coming from the plug instead of just setting a new buck converter to whatever the plug says?

600V DC = 009
200V DC = 10.1
20V DC = 10.21

I was expecting the number to go down with the lower Voltage setting but it went up so why don't I get a reading of 8.4v from the plug?

Thanks

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flippy   10 kW

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Re: Multimeter - Testing/Results

Post by flippy » Jun 18 2018 3:08am

you set the voltage on the meter without going over. so anything below 20V you set it to 20V. 21V you set it to 200V range. you lose a digit that way but it can read it. if you want to measure lower voltages you can set it to 600V if you want to. the resolution and accuracy is completly out to lunch at that point but you could.

a meter works with "counts". this one is 2000 counts. real crappy but it does the job.

2000 counts means the mm can only display up to 20V with 2 zero's (so 20.00V). so at the 200V range you lose another digit and so on.
a more capable meter would have 6000 counts or more and usually have a 6V range for example. that way you can read a battery up to 3 decimal places. (4.200V)
i am not getting into the whole accuracy thing but 2000 counts is really crappy for battery measurements because the voltage is way too low.

for reference: if you go balls out you can get 500.000 count meters.

see our mighty lord and saviour dave's video for more info on this: https://www.youtube.com/watch?v=U4JFeU-o2kc
Last edited by flippy on Jun 18 2018 6:21am, edited 1 time in total.
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Re: Multimeter - Testing/Results

Post by amberwolf » Jun 18 2018 3:47am

toxictoad wrote:
Jun 16 2018 12:15pm
I am still confused as to why the MM readings from the plug don't match 8.4v
You said that MM read 8.40v--then that is exactly the same as the voltage you are looking for in the sentence quoted above.

If instead you mean that the MM is reading the ~10v, then that's probably the "unloaded" voltage (without anything plugged in).

If it is not the unloaded voltage, and is instead with the light plugged into it, then that simply means the wallwart it came with is not well-regulated. (or is defective, but probably not).


As for the smoke coming from the light, if that happened with the converter rather than it's original wall power supply, it may mean the converter put out too high a voltage, so somethng overheated and burned. It might nto be an important part required for operation, but it might be a protection part intended to prevetn power spikes/etc from damaging the unit--if so, the unit is probably now unprotected against such things.

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Re: Multimeter - Testing/Results

Post by toxictoad » Jun 28 2018 6:53am

Thanks for the info flippy and amber,

I think I'll always struggle to understand but I'm picking things up slowly and hopefully everything will work out safe and sound :)

flippy, I get what you're saying in your first paragraph and easier for me to remember, if I move onto another project I'll think about another MM but for what I use it for...

amber, the MM didn't register 8.40v (apart from out of the converter after I set it. The plug says 8.4v.

The test I did was to put the plug into the mains as normal but with bare wires touched MM connectors to wires and turned on

Based on what flippy said (if I got it right) the closest reading is 10.21 @ 20V DC but now I guessing that this is the unloaded voltage yeah?
If that's the case then would it been better to expose the wires but not disconnect the device so that I can test under load? and doing that would have given me 8.4v?

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Re: Multimeter - Testing/Results

Post by toxictoad » Jun 28 2018 7:03am

I'm wanting to connect 4 x step down converts (linked before) and am looking for a junction box, would this be a good option?
20 AMP 4 Terminal

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